Add SeaweedFS-backed file storage infrastructure

Introduces IFileStorageProvider and an S3-compatible implementation (S3FileStorageProvider) configured via new SEAWEEDFS_* configuration keys, for storing uploaded files against SeaweedFS.

web/backend/Infrastructure/Infrastructure.FileUpload/IFileStorageProvider.cs

@@ -0,0 +1,29 @@
+namespace Infrastructure.FileUpload;
+
+/// <summary>
+///     Service for storing and retrieving uploaded files in S3-compatible object storage.
+/// </summary>
+public interface IFileStorageProvider
+{
+    /// <summary>Uploads a file to the configured bucket.</summary>
+    /// <param name="key">The object key (path) to store the file under.</param>
+    /// <param name="content">The file content.</param>
+    /// <param name="contentType">The MIME type of the file.</param>
+    /// <param name="cancellationToken">A token to cancel the upload.</param>
+    Task UploadAsync(
+        string key,
+        Stream content,
+        string contentType,
+        CancellationToken cancellationToken = default
+    );
+
+    /// <summary>Deletes a file from the configured bucket.</summary>
+    /// <param name="key">The object key of the file to delete.</param>
+    /// <param name="cancellationToken">A token to cancel the deletion.</param>
+    Task DeleteAsync(string key, CancellationToken cancellationToken = default);
+
+    /// <summary>Generates a time-limited URL for retrieving a file directly from storage.</summary>
+    /// <param name="key">The object key of the file.</param>
+    /// <param name="expiresIn">How long the URL remains valid.</param>
+    string GetPresignedUrl(string key, TimeSpan expiresIn);
+}